Photodetectors, devices that convert photons to electricity, are widely used in digital imaging, optical communications, remote sensing, night-time surveillance, medical imaging, and so on. Their sensitivity, the ability to differentiate signal from noise, is key for high-fidelity photon detection and imaging, especially when the signal is weak. To achieve high sensitivity, a high gain is needed to amplify the signal far above the noise baseline.
One way to achieve high gain is to multiply the photogenerated charge carriers in a single carrier transport and collection cycle, as done in photomultiplier and avalanche photodiode devices, which achieve typical gains of 103-108 carriers per incident photon. However, for photomultiplier and avalanche photodiode devices, the required high bias (hundreds to thousands of volts) and their bulky nature restrict their integration with micro-electronics for digital imaging. In addition, the electron multiplication processes give rise to excess noise.
Another approach towards high gain is to collect each photocarrier multiple times over many transport cycles in simple, two-terminal devices with semiconductor channels. These devices, known as photoconductive material-based devices (a subset of photodetectors), are designed to trap the minority charge carriers for a long time, enabling majority carriers to recirculate through the device many times before recombining. In this way, multiple carrier collection occurs with the absorption of one photon. Small and simple in design, photoconductive material-based devices are compatible with modern micro-electronics, and can be integrated as, for example, pixel sensors in the widely used CMOS (complementary metal-oxide-semiconductor) technologies.
The performance of a photoconductive material-based device depends on the deliberate control of minority carrier trapping, with the goal of achieving long carrier lifetime while preserving the high-mobility, low-noise majority carrier transport. Typically, minority carriers are retained in sub-bandgap states or electrostatic barriers induced by defects, dopants, electronic junctions, or a combination of these factors. However, these minority carrier traps are often in the pathway of majority carrier transport, leading to carrier scattering, reduced mobility, and noise, which limit the photon sensitivity.